# include <iostream>
# include <fstream>

using namespace std;

const int MAX_N = 100000;
int x[MAX_N], y[MAX_N];

int main() {
    ifstream fin( "aria.in" );
    ofstream fout( "aria.out" );

    int n;
    fin >> n;

    for ( int i = 0; i < n; i ++ )
		fin >> x[i] >> y[i];

	long long s = 0;
	for ( int i = 0; i < n; i ++ )
		s += 1LL * ( x[i] + x[(i + 1) % n] ) * ( y[i] - y[(i + 1) % n] );

	fout << abs( s ) * 0.5;

	fin.close();
	fout.close();

    return 0;
}